Pour in the milk and stir, using a wooden spoon, until just mixed and there are no visible traces of flour. Gently fold in the grapes. Spoon into the prepared muffin tins and sprinkle tops with an even amount of coarse sugar, if desired.

Bake 20 minutes, until tops are golden and a skewer inserted comes out clean (it's okay if there's grape jam on it). Remove from oven and let cool 10 minutes in the tin, then transfer to a wire rack to cool completely before serving.